Auditory and Vestibular systems Flashcards
Describe the basic structure of a hair cell
• The hair bundles are filled with actin – they are stiff and rigid.
• These hair bundles sit on top of a hair cell which has a synapse, a sensory nerve fibre which then projects to the brain.
• So, the hair converts motion of these stereo cilla into neurotransmitter which is converted into electrical activity in the brain
• The hair cells also have a extracellular matrix
Stereocilia bundles
How are stereocilia arranged?
• Stereocilia are arranged in ‘bundles’ (e.g. 30-300 stereocilia in each bundle in the ear)
• Within the bundle stereocilia can be connected via a number of links:
o Connectors: Lateral-link, top connectors, shaft connectors and ankle links.
o Tip links: Found at the top of the cilia
What is the function of the lateral link connecters?
• The lateral link connectors bundle the hair bundles together – they hold them together and help them move in the same direction at the same time
What does tension in the tip link cause?
- Tension in the ‘Tip-links’ distorts the tip of the stereocilia mechanically
- This distortion allows channels to open and close with cilia movement. Current flows in proportionately. Positively charged ions move into the stereo cilla
- If the hair bundle moves in the opposite direction the top links do not distory the stereocilla and ion channels do not open
How is the neurotransmitter released in a hair cell?
- Tip-links’ open ion-channels.
- Endolymph high in K+.
- Potassium ion (K+) influx depolarises the cell.
- Voltage gated Ca2+ channels open.
- Ca2+ triggers neurotransmitter release at the synapse.
- Post-synaptic potential in nerve fibre triggers an action potential.
How can fish and amphibians detect water movement?
- Most fish and amphibians have a lateral line system along both sides of their body.
- They are Mechanoreceptors that provides information about movement through water or the direction and velocity of water flow.
- Important for schooling.
What encases the hair cell bundle in the ear and what does it detect?
- Superfical neuromasts are on the surface.
- Neuromasts function similarly to mammalian inner ear.
- A gelatinous cupula encases the hair cell bundle and moves in response to water motion.
What is the inner ear formed from?
What is the auditory system called?
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Cochlea
Describe the orientation and motion in mammals?
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What do semicircular canals sense?
sensing rotation
- Rotation causes fluid motion in the semicircular canals.
- Hair cells at different canals entrances register different directions.
How do hair cells sense rotation?
- Cilia are connected to the gelantinous cupula.
- Under motion, fluid in the canals lags to due to inertia, pulling the cupula in the opposite direction to the rotation of the head.
- Cilia are displaced, depolarising haircells.
What do otolith organs detect?
- In the otolith organs they are sensitive to linear acceleration.
- Gravity is also acceleration.
How do otolith organs work?
- Hair cells are topped by a rigid layer of otoconia crystals.
- Under acceleration the crystal layer is displaced, deflecting the cilia.
Describe the auditory pathway
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What is sound?
- Variation in pressure of air molecules
- Longitudinal pressure waves in the atmosphere.
- Imagine a slinky spring being pushed and pulled along its length.
What determines the wavelength and frequency
What equation links these two things?
The rate at which the compression and rarefaction of a wave occur determine the distance between two peaks in the wave (wavelength) and the rate at which the pressure cycles between compression and rarefaction (frequency).
Frequency and wavelength are inversely related. λ = c/f c = speed of sound (344m/s) f = frequency λ = wavelength
What’s the normal air pressure and what changes inair pressure can we detect?
- Normal air pressure: 100k Pascals.
* We can hear a .000000001% change in pressure.
Have a look at the sound pressure level
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What’s the function of the pinna?
What is it made from?
- Size and shape varies from person to person.
- Gathers sound from the environment and funnels it to the eardrum.
- Made entirely of cartilage and covered with skin.
- The outer ear filters, influencing the frequency response.
- Pinna features influence the entering sound differently.
What are the 4 grades of Microtia?
Grade I: A less than complete development of the external ear with identifiable structures and a small but present external ear canal
Grade II: A partially developed ear (usually the top portion is underdeveloped) with a closed stenotic external ear canal producing a conductive hearing loss.
Grade III: Absence of the external ear with a small peanut-like vestige structure and an absence of the external ear canal and ear drum. Grade III microtia is the most common form of microtia (see photo).
Grade IV: Absence of the total ear or anotia.
What is the ossicles?
What does the malleus connect do?
What is the function of the incus?
- Smallest bones in the human body.
- Connects the tympanic membrane to the oval window of the cochlea.
- Malleus connects to the ear drum
- The incus acts as a leaver for the malleus to amplify sound
What is glue ear?
- Middle ear fills with fluid which impedes motion of the ossicles.
- Reduces middle ear gain, raises hearing thresholds.
- Very common in small children (<5 yrs) - can lead to development problems.
- Fluid is present instead of air, so you lose amplification and the ability to hear, is can lead to developmental problems as it occurs in children
What is the function of the stapes?
What are the 3 chambers of the cochlea?
- Here the stapes is pushing on cochlea of ear
- There are 3 chambers of the cochlea as seen in diagram
- Stapes causes compression of the cochlea
- It is very rigid at the start and compliant at the start. The more stiff is resonates at a high frequency and at the end it
Describe the structure of the cochlea
• The cochlea: fluid filled spiral canal divided by a flexible membrane.
What does the Basilar membrane do?
• Basilar membrane filters sound according to frequency.
Where does the sound wave in the staples occur
Peak location depends on stimulus frequency.
What is the the organ of corti?
- The organ of corti sits on top of the basliar membrane, within the scala media.
- Inner and outer hair cells are mounted on it.
- The membrane can move up and down.
- This pushes hair cells towards the tectorial membrane, pushing them sideways opening the ion channels, causing firing of the nerves
- Motion of the organ of corti on the basilar membrane causes displacement of the stereocilia.
- Outer hair cells contact the tectorial membrane. Inner hair cells do not.
What do the inner and outer hair cells function as?
The inner hair cells are responsible for the transduction the outer act as an amplifier
What causes the outer hair cells to contract?
- Outer hair cells are motile.
- Influx of positive ions makes the outer hair cells contract
- Increases in voltage make the cell contract and decreases in cell voltage make it expand
How is sound amplified?
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Describe the ‘battery’ driving cochlear hair cells
- The high potassium concentration of the endolymph of the scala media creates a 2x amplification.
- If it were not potassium rich then inner hair cell output (of the cochlea nerve) would be halved, making sound perceptually quieter.
- And the cochlea amplification would be much smaller, again making sounds perceptually quieter.
